White light illumination may be realized by using blue Light Emitting Diodes (LEDs) in combination with a wavelength converting layer, such as a phosphor layer, that absorbs part of the blue light and converts it to colours with a longer wavelength. In the output light the different wavelengths are combined, resulting in white light.
However, there are a number of problems associated with this approach. One problem stems from the fact that the output light from the wavelength converting layer will have different colour compositions in different directions. In particular, the output light emitted at small angles has typically a higher proportion of blue light compared to the light emitted at large angles which typically has a higher proportion of yellow light. This is sometimes referred to as the “yellow ring problem”.
US2010064177 A discloses a LED assembly which aims at solving the yellow ring problem without reducing the efficiency of the LED assembly. The assembly comprises a LED die, a phosphor layer, and a filter layer. The filter layer is developed in such a manner that light rays with a wavelength of about 400 nm to 500 nm, preferably of about 420 nm to 490 nm, emitted from the LED die are at least partially reflected depending on their emission angle to the normal on the filter layer.
The wavelength converting layer may be placed on top of the LED chip. However, in order to achieve higher efficiencies and less colour shifting due to thermal effects, the wavelength converting layer is often placed at a distance from, or remote from, the LED chip. As a result of the wavelength converting layer being located at a distance from the LED, the size of the wavelength converting layer is larger than the light emitting part of the LED. A drawback with LED light sources having a remote wavelength converting element is that the emitted light is more difficult to collimate with a compact system than light of smaller light sources such as white LEDs.